In a retainer type lead-acid battery, a retainer such as a mat-like separator retains an electrolyte solution, and oxygen and hydrogen generated during charging of the lead-acid battery are returned to water by gas recombination. Patent Document 1 discloses that in a retainer type lead-acid battery, accumulation of lead sulfate in a negative electrode material easily proceeds when a ratio of a theoretical capacity of the negative electrode material to a theoretical capacity of a positive electrode material is set to less than 1. Further, Patent Document 1 discloses that the accumulation of lead sulfate can be suppressed to improve life performance by including 0.4% by mass or more of carbon in the negative electrode material.
Patent Document 2 discloses to make a negative active material (negative electrode material) of the retainer type lead-acid battery contain bisphenol/aminobenzenesulfonic acid/formaldehyde condensate and flake graphite. Patent Document 2 discloses that the bisphenols condensate is an expander of the negative active material and more improves charge acceptance than lignin which is an existing expander. Further, Patent Document 2 discloses that the bisphenols condensate reduces a hydrogen overvoltage, but when a storage battery is used in a partial state of charge (PSOC), a problem does not arise since generation of hydrogen does not occur. Moreover, Patent Document 2 discloses that flake graphite is a carbon-based electrical conductive material which improves charge acceptance of the negative active material.